Edge distance effects on the tensile behaviour of screw anchors installed in early age concrete: Experimental research and predictive model

Abstract

Screw anchors are increasingly used for temporary applications and are therefore implemented in both structural and non-structural capacities, such as: fixing of temporary safety handrails or barriers to concrete slabs in multistorey constructions, connection of scaffolding to the slab edge running up the face of a building during construction or connection of prop system to the concrete slab for formwork assembly. Current design codes, based on the Concrete Capacity Method (CCM), assume a mature concrete cone failure mode, which is highly sensitive to edge distance. This study investigates this assumption through an experimental program of 24 pull-out tests on M10 anchors in concrete at 24h, 48h, 7 days, and 28 days, at edge distances of 40mm, 60mm, and 90mm. The findings reveal a fundamental shift in the failure mechanism: all early-age samples failed via pull-out failure, irrespective of edge distance. This is attributed to low concrete strength being insufficient to activate cone failure. Consequently, the pull-out mode, which is independent of edge proximity, becomes the governing limit state, rendering edge distance insignificant in early-age applications and contradicting CCM-based models. This study further demonstrates that existing pull-out models significantly overestimate capacity in early-age concrete. Therefore, a new predictive model for pull-out failure is proposed, recalibrating the existing model's calibration factor (k=15.6) based on the early-age experimental data to improve prediction accuracy. The conclusions drawn in this study are therefore restricted to this anchor configuration, installed in normal-strength N40 concrete at ages of 24 h, 48 h, 7 days and 28 days and at edge distances of 40–90 mm, and should be interpreted within this specific range of test conditions.